This invention relates to a microwave monolithic integrated circuit (MMIC) assembly and, more particularly, to such an MMIC assembly wherein the MMIC is supported on a heat-dissipating assembly having multiple pieces of pyrolytic graphite with their high-thermal-conductivity x-directions oriented for optimal heat dissipation from the MMIC.
A microwave monolithic integrated circuit (MMIC) is a microwave circuit in which one or more discrete microwave devices are mounted on a substrate. External connections and interconnections between the devices are provided on the substrate. The connections are provided both for low-frequency signals and for the microwave signals being processed. The microwave devices in the MMIC may be of any type.
In a power amplifier or other high-power MMIC, the microwave devices include microwave circuits that process a high-power microwave signal. A large amount of heat is generated as a by-product of the microwave signal processing. The heat must be redistributed and ultimately conducted away, or the resulting increased temperature may exceed the maximum operating temperature limit of the microwave device. If the maximum operating temperature limit is exceeded, the performance of the microwave device is degraded or the device could fail.
The MMIC may be mounted on a heat-management structure that facilitates the initial stages of the removal of the heat from the microwave devices and the substrate to which they are mounted. Historically, the heat-management structure was made of a ceramic such as aluminum oxide, a metal, or a composite material. As the heat outputs have risen and the sizes of the microwave devices have been reduced, the available heat-management materials have not provided the required heat-removal capabilities.
More recently, it has been proposed to utilize encapsulated pyrolytic graphite as the heat-management material. Pyrolytic graphite is an anisotropic material having a high-thermal-conductivity x-direction in which the thermal conductivity is at least 5-10 times greater than many alternative heat-management materials. Pyrolytic graphite also has a low thermal expansion coefficient, reducing the differential thermal strains and stresses between the heat-management structure and the MMIC.
Although pyrolytic graphite offers advantages for use as a heat-management material, it has not been optimized for use with devices such as the MMIC assembly. There is therefore a need for a design in which the pyrolytic graphite is optimized for use in the MMIC assembly, so that its potential may be more fully realized in dissipating heat and maintaining the MMIC within its operating temperature limit. The present invention fulfills this need, and further provides related advantages.
The present invention provides a microwave monolithic integrated circuit (MMIC) assembly in which encapsulated pyrolytic graphite is used as a heat-dissipation material underlying the MMIC substrate. The spatial orientations of the pyrolytic graphite core are selected for optimal dissipation of heat, recognizing the spatial variation in heat production by the MMIC. The heat-dissipation assembly is readily fabricated as a closed, integral unit that is highly resistant to oxidation, corrosion and other adverse environmental influences.
In accordance with the invention, a microwave monolithic integrated circuit (MMIC) assembly comprises a microwave monolithic integrated circuit lying in an MMIC circuit plane. The MMIC has a first region of relatively high heat production and a second region of relatively low heat production. The first region typically corresponds to the location on the MMIC substrate of a high-heat-output device such as a power amplifier.
A heat-dissipating assembly is in thermal contact with the MMIC. The heat-dissipating assembly has a core comprising at least two pieces of pyrolytic graphite embedded within a casing and bonded to an interior wall of the casing. The pieces of pyrolytic graphite comprise a first piece of pyrolytic graphite underlying (i.e., in vertical alignment with) the first region of relatively high heat production and having a high-thermal-conductivity x-direction of the first piece lying within about 20 degrees of a perpendicular (and preferably substantially perpendicular) to the MMIC circuit plane, and a second piece of pyrolytic graphite underlying the second region of relatively low heat production and having a high-thermal-conductivity x-direction of the second piece lying within about 20 degrees of (and preferably substantially parallel to) the MMIC circuit plane.
The microwave monolithic integrated circuit may include multiple first regions and multiple second regions. In that case, the heat-dissipating assembly includes multiple first pieces of pyrolytic graphite underlying the respective multiple first regions, and multiple second pieces of pyrolytic graphite underlying the respective multiple second regions. The heat-dissipating assembly may further include one or more third pieces of pyrolytic graphite that do not correspond to and underlie the first region of the MMIC, but which have the high-thermal-conductivity x-direction of the pyrolytic graphite within about 20 degrees of the perpendicular (and preferably substantially perpendicular) to the MMIC plane.
In the MMIC assembly, the casing is preferably a metal such as aluminum, copper, and silver, and alloys thereof. The casing preferably comprises a first preform contacting a top of the core, a second preform contacting a bottom of the core, and a lateral wall enclosing a lateral periphery of the core. The casing may be hermetic or non-hermetic. A hermetic casing is preferred, to protect the pyrolytic graphite against environmental attack. The heat-dissipating assembly desirably has no structural layers that are organic materials. Minor amounts of organic contaminants may be present without adversely affecting the functionality of the heat-dissipating assembly, but there are no layers or structural elements made of organic materials intentionally present in the heat-dissipating assembly.
A method for fabricating a microwave monolithic integrated circuit (MMIC) assembly comprises the steps of furnishing a microwave monolithic integrated circuit lying in an MMIC circuit plane and having a first region of relatively high heat production and a second region of relatively low heat production. Separately, a heat-dissipating assembly is fabricated which has a relatively large dimension lying in a heat-dissipating-assembly plane and a relatively small dimension lying perpendicular to the heat-dissipating-assembly plane. The heat-dissipating assembly has a core comprising at least two pieces of pyrolytic graphite embedded within a casing and bonded to an interior wall of the casing. The pieces of pyrolytic graphite comprise a first piece of pyrolytic graphite having a high-thermal-conductivity x-direction of the first piece lying substantially perpendicular to the heat-dissipating-assembly plane, and a second piece of pyrolytic graphite having a high-thermal-conductivity x-direction of the second piece lying substantially parallel to the heat-dissipating-assembly plane. The microwave monolithic integrated circuit is thereafter assembled to the heat-dissipating assembly with the MMIC circuit plane parallel to the heat-dissipating-assembly plane and with the first piece of pyrolytic graphite underlying the first region of relatively high heat production and the second piece of pyrolytic graphite underlying the second region of relatively low heat production. Other features as discussed above may be utilized in relation to this method.
The fabricating of the heat-dissipating assembly preferably includes furnishing the two pieces of pyrolytic graphite and a set of disassembled elements of a casing, assembling the pieces of pyrolytic graphite within the interior of the disassembled elements of the casing positioned so as to form an initial assembly, placing the initial assembly into an evacuated interior of an elevated-temperature pressing apparatus, and heating and simultaneously applying pressure to the initial assembly using the elevated temperature pressing apparatus until a resulting heat-dissipating assembly is substantially fully dense. This heating-and-applying pressure step is desirably accomplished by hot isostatic pressing.
The present approach places the first pieces of the pyrolytic graphite, with the high-thermal-conductivity x-direction near to perpendicular to the MMIC circuit plane, underlying the first regions of the MMIC that have the highest heat production. Heat dissipation from these first regions is thereby facilitated. The second pieces, in which the high-thermal-conductivity x-direction lies near to parallel to the MMIC circuit plane, dissipates heat laterally so that the heat is may be more readily conducted out of the heat-dissipating assembly. The pyrolytic graphite has a low coefficient of thermal expansion in both the x-direction and a z-direction lying perpendicular to the heat-dissipating-assembly plane.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.